AI: Clone the wrapped builder when cloning WP_AI_Client_Prompt_Builder

src/wp-includes/ai-client/class-wp-ai-client-prompt-builder.php

@@ -223,6 +223,23 @@ public function __construct( ProviderRegistry $registry, $prompt = null ) {
 		);
 	}
 
+	/**
+	 * Clones the wrapped prompt builder alongside this instance.
+	 *
+	 * The wrapped builder mutates its own state, so without this a clone would
+	 * share that state with the original and any change made to one would be
+	 * visible in the other.
+	 *
+	 * @since 7.2.0
+	 */
+	public function __clone() {
+		$this->builder = clone $this->builder;
+
+		if ( null !== $this->error ) {
+			$this->error = clone $this->error;
+		}
+	}
+
 	/**
 	 * Registers WordPress abilities as function declarations for the AI model.
 	 *

tests/phpunit/tests/ai-client/wpAiClientPromptBuilder.php

@@ -2613,6 +2613,75 @@ static function ( $prevent, $builder ) use ( &$captured_builder ) {
 		$this->assertInstanceOf( WP_AI_Client_Prompt_Builder::class, $captured_builder );
 	}
 
+	/**
+	 * Returns the text of every message part held by a prompt builder.
+	 *
+	 * @param WP_AI_Client_Prompt_Builder $builder The prompt builder to read.
+	 * @return string[] The text of each message part, in order.
+	 */
+	private function get_prompt_parts( WP_AI_Client_Prompt_Builder $builder ): array {
+		$wrapped = new ReflectionProperty( WP_AI_Client_Prompt_Builder::class, 'builder' );
+		self::set_accessible( $wrapped );
+		$inner = $wrapped->getValue( $builder );
+
+		$messages = new ReflectionProperty( $inner, 'messages' );
+		self::set_accessible( $messages );
+
+		$parts = array();
+		foreach ( $messages->getValue( $inner ) as $message ) {
+			foreach ( $message->getParts() as $part ) {
+				$parts[] = (string) $part->getText();
+			}
+		}
+
+		return $parts;
+	}
+
+	/**
+	 * Tests that a clone does not share the wrapped builder with the original.
+	 *
+	 * @ticket 65782
+	 */
+	public function test_clone_does_not_share_the_wrapped_builder() {
+		$builder = new WP_AI_Client_Prompt_Builder( AiClient::defaultRegistry(), 'Original prompt' );
+		$clone   = clone $builder;
+
+		$wrapped = new ReflectionProperty( WP_AI_Client_Prompt_Builder::class, 'builder' );
+		self::set_accessible( $wrapped );
+
+		$this->assertNotSame(
+			$wrapped->getValue( $builder ),
+			$wrapped->getValue( $clone ),
+			'A clone should wrap its own builder instance'
+		);
+
+		$clone->with_text( 'Added to the clone' );
+
+		$this->assertSame( array( 'Original prompt' ), $this->get_prompt_parts( $builder ), 'Changing the clone should not change the original' );
+	}
+
+	/**
+	 * Tests that the clone passed to the prevent prompt filter cannot change the prompt.
+	 *
+	 * @ticket 65782
+	 */
+	public function test_prevent_prompt_filter_cannot_mutate_the_original_prompt() {
+		add_filter(
+			'wp_ai_client_prevent_prompt',
+			static function ( $prevent, $builder ) {
+				$builder->with_text( 'Added by the filter' );
+				return $prevent;
+			},
+			10,
+			2
+		);
+
+		$builder = new WP_AI_Client_Prompt_Builder( AiClient::defaultRegistry(), 'Original prompt' );
+		$builder->is_supported();
+
+		$this->assertSame( array( 'Original prompt' ), $this->get_prompt_parts( $builder ), 'A filter should not be able to change the prompt' );
+	}
+
 	/**
 	 * Tests that once in error state, subsequent fluent calls return the same instance.
 	 *